Technical Field                The present disclosure relates generally to carbon dioxide (CO2) leak detection and more specifically to methods and apparatus for measuring leaks from a site, such as a CO2 sequestration facility.        
Background Information
Since the beginning of the industrial revolution, the human race has been burning larger and larger quantities of fossil fuels and emitting greater and greater amounts of CO2 into the atmosphere in the process. The atmospheric burden of CO2 has increased dramatically and continues to rise. This causes more heat to be trapped at the Earth's surface, driving surface temperatures higher. Further, significant quantities of excess CO2 are dissolving in the oceans, making them more acidic. In general, the release of CO2 into the atmosphere is driving dangerous global climate change.
One proposed approach for mitigating the atmospheric burden of CO2 is long term carbon capture and storage (CCS). In this approach, CO2 is captured, concentrated, and then stored in a sequestration facility, rather than being emitted into the atmosphere. Typically, storage involves pumping the concentrated CO2 into underground geologic formations, where it is retained. In order to provide benefits, the CO2 needs to be retained for thousands of years. During such time, it is important for both human safety and to achieve the intended environmental benefits that any leakage from the facility be detected and addressed. Such leakage may be quite small (e.g., 0.01% per year) and potentially dispersed across a wide area of ground above the facility. Even small, dispersed leaks are troublesome, as they can significantly reduce the climate change benefits of CCS.
Unfortunately, detecting (and potentially quantifying) small, dispersed leaks of CO2 is quite challenging. One reason why such leak detection is challenging is the naturally high variability of CO2 at the Earth's surface. CO2 is continuously produced and consumed by a wide variety of ambient processes, such as fossil fuel combustion, photosynthesis, plant and soil respiration, etc. These local influences can lead to significant variations in the “natural” concentration of CO2 close to ground level (e.g., variances of several parts per million (ppm) over a short period of time). It is difficult to detect small, dispersed leaks of CO2 from a sequestration facility in the presence of this interference caused by “natural” processes.
One approach that has been investigated for detecting leaks of CO2 is to mix a chemical tracer with the CO2 that has a low background concentration and low natural variability, for example, a perfluorocarbon or CO2 isotope. However, this approach has a number of disadvantages, which may include (depending on the tracer chosen), expense of the tracer, adverse environmental effects of the tracer, and potential for the tracer to move differently through geologic formations than the sequestered CO2.
Another more promising approach involves simultaneously measuring atmospheric oxygen (O2) in conjunction with CO2 to discriminate between leaks of CO2 from a sequestration facility and interference caused by “natural” sources. Changes in CO2 and O2 concentrations resulting from natural processes are generally anti-correlated. For example, in combustion O2 is consumed and CO2 is released. Likewise, in photosynthesis, CO2 is consumed and O2 is released. In contrast, there is generally little or no anti-correlation with O2 concentration when a change in CO2 concentration is the result of a leak from a sequestration facility.
Unfortunately, it is difficult to simultaneously measure O2 in conjunction with CO2 as required by this approach, in part because detecting O2 at an optimal speed and at an optimal accuracy is challenging. One known technique for measuring O2 and CO2 involves separate O2 and CO2 analyzers. In this technique, the O2 analyzer may use two fuel cells that include a lead anode and a gold cathode, one cell operating as a sample cell and the other as a reference cell. The current that is generated by the chemical reaction in each cell is linearly proportional to the partial pressure of the O2 concentration in the cell. The separate CO2 analyzer may use a non-dispersive infrared (NDIR) photometer to determine CO2 concentration. The NDIR may have a single path, which is alternatively used with sample gas and a reference gas.
However, this existing technique for measuring O2 and CO2 has a number of shortcomings which has hindered its widespread deployment. First, this existing technique has insufficient measurement time resolution to observe concentration fluctuations that occur on time scales of less than one second to several seconds (it is only capable of a measurement every 3 minutes). Second, it requires elaborate calibration procedures and comparisons to reference standards. These requirements add cost, labor and the potential for installation errors. Third, it requires very near complete removal of naturally-occurring water vapor from the examined sample (e.g., removal to <1 parts per million by volume (ppmv)). This typically requires multiple stages of water removal structures, and operations such as cryogenic trapping, significantly complicating instrument design and operation, and requiring periodic replenishment of consumables. Fourth, it is generally unsuited for sampling at multiple locations across a sequestration facility, in part, because the calibration and water removal requirements, together with the use of separate O2 and CO2 analyzers, leads to a complex and expensive inlet assembly.
Accordingly, there is a need for improved methods and apparatus for measuring leaks from a site (e.g., a CO2 sequestration facility) that may address some or all of these shortcomings.